Treatment of acid setting baths used in artificial silk manufacture



Patented Apr. 3, i93

lddlttd (3F ACID SETTING BATES USED EN ARTEFEQIAL TUBE SILK MANUFAQ- No Drawing, Application February 3, 1931, Serial No. 513,158

5 Claims.

The subject matter of this invention is the treatment, of the acid setting baths used in artificial silk manufacture, and more especially those employed in the so-called viscose-rayon process.

In artificial sill: manufacture, it is the practice to spin a solution of cellulose through a spinneret into an acid settin bath which sets hardens the solution as a bundle of filaments known as the thread or yarn of artificial silk. As the spinning operation proceeds, colored colloidal impurities gradually accumulate in the old setting bath; and after awhile the bath becomes so impure that it must be renewed in order to avoid undesirable coloration and contamination of the silk yarn. It is not feasible to remove such impurities by filtration or settling, because of their highly colloidal character. In fact, these impurities do not tend to settle out to the desired degree, even after long standing; and they tend to pass through filters and to clog the channels of filtration.

After considerable experimentation, we have discovered that contaminated acid setting baths may be readily and practically completely clarified by the simple expedient of first heating, preferably to about the boiling point, and then applying the usual methods of separating a solid from a liquid, via, filtration, centrifugation, and settling. The initial heating step causes a visible coagulation of the colloidal impurities, perhaps as a result of nemical decomposition and/ or physical aggregation. In any event, these impurities are transformed to a condition where they may be easily and practically completely removed by the usual methods.

The present invention has been found to work with eminent success especially in the case of the acid setting solutions employed in the manufacture of viscose-rayon. The setting baths employed in the viscose-rayon process usually consist of an aqueous solution of sulphuric acid, sodium sulphate, and a relatively small amount of zinc or magnesium sulphate. When a solution of this type is employed, we have found that it may be desirable to add to the contaminated solution undergoing the treatment of the present invention a relatively small amount of oxidizing agent, such as ammonium persulphate, potassium perchlorate, hypochlorite bleach, chlorine, sodium peroxide, or hydrogen peroxide. When this addition is made, there is, particularly in the case of the stronger oxidants, a decolorizing action to some extent on th solution. When the solution is then heated to the boiling point, complete decolorization rapidly ensues, and this is attended by the formation of a colorless precipitate which resembles and quite likely is made up largely of sulphur. This indicates that the contaminations probably exist in the setting solution, both as free and as sulphur combined as oxidizablc, colored, organic-sulphur-co1np1exes. We have found that the colorless precipitate redissolves when heating of the solution is continued, perhaps as a result of the oxidation of the sulphur into colorless salts. We may hence eliminate the precipitate from the solution either physically as by filtration, or chemically by continued heating of the solution.

In actual practice, one for example, be using an aqueous solution containing approximately 9% sulphuric acid, 22% sodium sulphate, and 1% zinc sulphate or magnesium sulphate for making up the baths into which the viscose syrup is spun and set as artificial silk. The use of the baths may be continued until they have picked up so much impurity that they have a yellow or orange color. At this stage, it is desirable to decolorize and clarify the bath solution, since otherwise the silk yarn formed therein will. be undesirably stained and contaminated. In accordance with the present invention, the contaminated solution is removed from the spinning vats and heated to the boiling point or thereabout, which temperature is maintained for about one hour, in order to ensure practically complete coagulation of the colloidal impurities. Coagulation actually starts at about 90 C. and is accompanied by the flotation of considerable of the coagulated material in pasty or mushy condition to the surface. This flotation is evidently brought about by the dissolved air present in the solution, which, being less soluble at high temperatures, gathers on the partially precipitated colloid and induces a large part of it to 95 rise to the surface just before the boiling point is reached. This scum may be removed during the heating period by skimming the solution. After coagulation has gone as far as it will go,

which, as already indicated, takes less than an 100 hour, the solution may be readily and completely separated from the solid impurities, as by filtration through an acid-resistant filtering medium, preferably of the type of sand or kieselguhr, as filters containing these media lend themselves to 105 backwashing and are readily restored to their original efficiency. Other filtering media, such as charcoal, may, however, be used. The solution is preferably still in hot condition when filtered,

in order to hasten its flow through the filtering Ill) medium. The solution issues in clear, practically water-White condition from the filter, and is thus ready for reuse in the setting vats.

The contaminated acid setting solutions resulting from viscose-rayon-making may contain an undesirable excess of SOdillIIl sulphate and a deficiency of sulphuric acid because of reaction of the sulphuric acid and the viscose to form sodium sulphate during the spinning and setting of the viscose. Hence, the clarified solution may need readjustment of its composition. To this end, the clarified solution, while still hot, may be delivered into evaporators wherein sufficient Water is removed from the solution to permit crystallizing out and recovery of the excess sodium sulphate. To the resulting solution may then be added sulphuric acid in amount to make up the loss of sulphuric acid, and Water in amount to restore the solution to its original composition.

lhe process of our invention obviates the loss of the chemicals used in acid setting baths of artificial silk mills and the nuisance apt to be created when the impure, acid setting solutions are disposed of in Waterways. It constitutes a simple, eifective, and inexpensive recourse in regenerating and using over and over again the contaminated acid setting solutions resulting from artificial silk manufacture. It is, of course, possible to depart in one or more respects from the process hereinbefore outlined, which to date is regarded as optimum, bearing in mind the low costs involved. For instance, it is possible, Without departing from the principles of my invention, to carry out the heating of the contaminated acid setting solution under pressure at, say, 130 0., or higher temperature. A process thus practised does not, however, yield. a clarified liquor offering any significant advantage over one which has been prepared by boiling under atmospheric pressure, as under such latter condition it is possible to coagulate and remove in short order 90% or more of the colloidal impurities.

What we claim is:

1. A process of regenerating the contaminated 'a colorless precipitate, and removing the precipitate.

2. A process of regenerating the contaminated acid setting solution resulting from viscoserayon-making, which comprises adding an oxidizing agent to the solution, heating the solution to cause its decolorization and the formation of a colorless precipitate, and continuing the heating of the solution until the precipitate has disappeared.

3. A process of regenerating a contaminated acid setting solution containing sulphuric acid and sodium sulphate and resulting from viscoserayon-making, which comprises heating the solution to at least about 90 C. for a sufiicient period of time .to coagulate substantially all colloidal impurities'present therein, removing the coagulated impurities, crystallizing out excess sodium sulphate formed by reaction of the sulphuric acid and the viscose during rayon-making, and adding sulphuric acid to make up the loss of sulphuric acid sustained by the reaction.

i. A process of regenerating the contaminated acid setting solution resulting from viscose- .rayon-making, which comprises adding an oxidizing agent to the solution and heating the solution to cause its decolorization and the formation of a colorless precipitate.

5. A process of regenerating .the contaminated acid setting solution resulting from viscoserayon-making, which comprises adding an oxidizing agent to the solution, heating the solution to cause its decolorization and the formation of a colorless precipitataand eliminating the precipitate.

GEORGE A. RICHTER. JOHN G. GOSSELINK. 

